Regeneration of used lubricant oil by propane extraction

Liquid and supercritical. propane has been used as a solvent for the recycling of used lubricant oils. The aim of the work has been to identify the best processing conditions to separate base oil suitable for the formulation of new lubricants, avoiding the coextraction of oxidation products and metallic compounds. The effect of pressure (30-60 kg/cm(2)) and temperature (20-140 degreesC) on the separation efficiency and yields has been investigated. In the pressure range analyzed, almost no effect of the variable on yields and metallic compounds removal was found. However, it did affect the separation of oxidation products that were removed more efficiently at low pressures. In regards to temperature, at a given pressure the extraction yields were found not to depend on the variable as long as propane remained as a liquid. However, when the temperature was increased at constant pressure, so propane became a gas or a supercritical fluid, extraction yields decreased. Furthermore, the yield decrease observed with supercritical propane was density-dependent: the higher the propane density, the higher the extraction yield. On the other hand, at constant pressure, metallic and oxidation compounds removal was found to increase with increasing temperature. Finally, propane-extracted oil at optimum conditions (30 kg/cm(2) and 90 degreesC) has been compared to two vacuum-distilled oils (5 mmHg), one of them pretreated with propane at optimum conditions.